1. Field of the Invention
The present invention relates to a new and improved blood flow measuring apparatus operating according to the H.sub.2 clearance measurement technique which comprises a measuring electrode, a reference electrode and a neutral electrode, and further contains an operational amplifier and a signal display device.
2. Discussion of the Background and Material Information
The blood flow through human and animal tissue is an important indicator both for diagnostics and also for the control of the progress and success of therapy or a surgical invasive procedure as well as for the trial and observation of the efficacacy of medication. Therefore numerous publications exist reporting experiences which have been had with measuring apparatuses for this purpose.
According to a prior art method for the determination of the blood flow through tissues a suitable radioactive material having a short half-life time is implanted in the tissue to be investigated. The decomposition products of this radioactive material preferably exhibit a still shorter half-life time and as a gaseous substance are dissolvable in blood. During the radioactive decomposition of the implanted material and the decomposition products there is produced radiation, the intensity of which can be measured with suitable instruments. The behavior of this intensity, in other words, the change of this behavior as a function of time allows for the quantitative determination of the decomposition products which have been washed out of the tissue by the blood flow, and thus, also enables determining the blood flow through the tissue.
The disadvantages of this method should be readily apparent. Very few radioactive materials are known which possess the previously considered properties and which can be produced with the required purity. The implantation of the radioactive material requires surgery and can not be carried out at every tissue or organ. Certain radioactive decomposition products tend to concentrate in certain tissues or organs and can cause protracted damage thereto. The instruments for measuring the intensity of the radioactive radiation of the implantation and the decomposition products detect a relatively large tissue volume, explaining why it is not possible to determined the blood flow through smaller tissue regions. Additionally, such instruments can only be used by trained personnel, are technically very complicated and therefore quite expensive.
According to a different prior art method, usually referred to as the H.sub.2 clearance measurement technique or method, the blood of the test subject or person is enriched with hydrogen and used as electrolyte forming a galvanic element in conjunction with two electrodes inserted into the tissue to be examined. The electrical potential of this galvanic element is governed, among other things, by the concentration of the hydrogen in the blood. When performing such H.sub.2 clearance measurement the hydrogen is introduced through the respiratory air or by injection into the blood. As soon as the electrical potential between the two electrodes has reached a certain threshold value there is interrupted the infeed of hydrogen and there is observed the decrease of the electrical potential as a function of time. The steepness or slope of the curve of this function constitutes a measure of the flow of blood through the tissue, during which the blood enriched with hydrogen is removed and replaced by hydrogen-free blood.
The theoretical basis for this H.sub.2 clearance measurement method, in particular the computation of the electrical potential of the electrodes as a function of the hydrogen-ion concentration using the Nernst equation and the determination of the flow of blood through a tissue volume by virtue of the decrease in the concentration of the hydrogen in the blood by means of the Fick principle or law, have been extensively described, for example, by Knut Aukland et al in Circulation Research, Volume XIV, 1964, pages 164 et seq. When performing this H.sub.2 clearance measurement method no implantation or complicated equipment is required. Additionally, this H.sub.2 clearance measurement method allows for repeated determinations of the blood flow through a tissue with altered conditions, and the comparatively small electrodes enable determining the blood flow through relatively small tissue volumes.
Notwithstanding these advantages and even though the determination of the flow of blood through tissues by means of an inert gas or hydrogen has been known for at least forty years and has been discussed in numerous publications, the practical application of this method has previously been essentially confined to experiments with animals and there is only known the measuring of the rate of blood flow which has been performed on the tip of the small finger of a human. The reasons for this are quite simple. With the apparatuses previously employed for the performance of the aforedescribed method there only could be accomplished measurements, which could be evaluated and were reproducible, if the current intensity between the electrodes, and thus, also in the blood and at the tissue to be investigated amounted to at least 1.multidot.10.sup.-6 Amp., a value which is physiologically questionable or even, in fact, impermissible for certain tissues.
Such a measuring apparatus has been described, for example, in the article: "H.sub.2 Clearance Measurement of Blood Flow: A Review of Technique and Polarographic Principles", authored by Wise Young, Ph.D., M.D.; in the publication "Stroke", Vol. 11, No. 5, September-October 1980, pages 552 to 564.